March for science

The Large Hadron Collider (LHC) is the world’s largest and most powerful particle accelerator till date. It started operating on 10 September 2008, and remains the latest addition to CERN’s accelerator complex upon further upgrades. It consists of a 27-kilometer ring of superconducting magnets with a number of accelerating structures to boost the energy of the particles along the way. Inside of its tunnel LHC steers subatomic particles to near the speed of light [2].

Why is it called the “Large Hadron Collider”?

"Large" refers to its size, approximately 27km in circumference; "Hadron" because it accelerates protons or ions, which belong to the group of particles called hadrons; "Collider" because the particles form two beams traveling in opposite directions, which are made to collide at four points around the machine.

The main goals

The Standard Model of particle physics – a theory developed in the early 1970s that describes and classifies all known fundamental particles and their interactions – has precisely predicted a wide variety of phenomena and most importantly successfully describes the three of the four known fundamental forces i.e electromagnetic, weak and strong interactions. But, despite of being self-consistence and huge successful in providing experimental predictions, the Standard Model is incomplete leaving many questions unexplained; for example: it fails to incorporate the fourth force - gravity and thus lacks to be a theory of all the fundamental interactions together, moreover it doesn't explain baryon symmetry, dark matter, neutrino oscillations etc. Now, the LHC along with few other next generation experiments around the globe come into the picture trying to investigate those unanswered phenomena [5].

What is the origin of mass?

The Standard Model does not explain the origins of mass, nor why some particles are very heavy while others have no mass at all. However, theorists Robert Brout, François Englert and Peter Higgs made a proposal that was to solve this problem. The Brout-Englert-Higgs mechanism gives a mass to particles when they interact with an invisible field, now called the “Higgs field”, which pervades the universe. Particles that interact intensely with the Higgs field are heavy, while those that have feeble interactions are light. In the late 1980s, physicists started the search for the Higgs boson, the particle associated with the Higgs field. In July 2012, CERN announced the discovery of the Higgs boson, which confirmed the Brout-Englert-Higgs mechanism. However, finding it is not the end of the story, and researchers have to study the Higgs boson in detail to measure its properties and pin down its rarer decays.

Figure 2. A cartoon representation of Higgs mechanism i.e the origin of mass [David Miller, UCL, 2013]

There are more tricky questions are open: Will we discover evidence for supersymmetry? What are dark matter and dark energy? Why is there far more matter than antimatter in the universe? How does the quark-gluon plasma give rise to the particles that constitute the matter of our Universe? [2].

History and Background

Many of us may think that the transformation from a vast green field to the largest particle physics laboratory in the world requires a leap of imagination, which is not wrong at all. Many imaginative leaps and jumps weave their ways through the story of CERN to make it what it is today. But a stroll through the past decades doesn’t just tell us the story of a laboratory, it also reflects the different challenges that grip particle physicists and the engineers [5].

Scientists started thinking about the LHC in the early 1980s, when the previous accelerator, the LEP, was not yet running. In December 1994, CERN Council voted to approve the construction of the LHC and in October 1995, the LHC technical design report was published [6].

But still it was challenging for the scientists to convince the engineers to build the then “impossible machinery”. It was reported that in early 90's the scientists and the advisory committee of CERN contacted the famous transport company Alstom for making those nearly impossible magnets. At that time Alstom just didn't take the proposal as realistic and turned down the contract. But in the later half of the decade, after much more research, plans and designing Alstom was convinced enough to be back into the game for giving those nearly impossible large and powerful magnets a reality [7].

Contributions from Japan, the USA, India and other non-Member States accelerated the process and between 1996 and 1998, four experiments (ALICE, ATLAS, CMS and LHCb) received official approval and construction work started on the four sites.

Components and Detectors

There are seven installed experiments at the LHC: ALICE, ATLAS, CMS, LHCb, LHCf, TOTEM and MoEDAL. They use detectors to analyse the myriad of particles produced by collisions in the accelerator. These experiments are run by collaborations of scientists from institutes all over the world. Each experiment is distinct, and characterized by its detectors.

The central part of the LHC is the world’s largest fridge. Thanks to the cryogenics at a temperature of 1.9 K (-271.3°C) ,colder than deep outer space, it contains important superconducting coils [6]. Inside the accelerator, two high-energy particle beams travel at close to the speed of light before they are made to collide. The beams travel in opposite directions in separate beam pipes – two tubes kept at ultrahigh vacuum (10^-6 mbar, whereas the standard atmospheric pressure is 1013.25 mbar). They are guided around the accelerator ring by a strong magnetic field maintained by superconducting electromagnets. If we added all the superconducting filaments(which is one tenth of the thickness of human hair) together, they would stretch to the Sun and back five times with enough left over for a few trips to the Moon [8]. The electromagnets are built from coils of special electric cable that operates in a superconducting state, efficiently conducting electricity without resistance or loss of energy. This requires chilling the magnets to -271.3°C – a temperature colder than outer space. For this reason, much of the accelerator is connected to a distribution system of liquid helium, which cools the magnets, and other linked supply services.

Thousands of magnets of different varieties and sizes are used to direct the beams around the accelerator. These include 1232 dipole magnets 15 metres in length which bend the beams, and 392 quadrupole magnets, each 5–7 metres long, which focus the beams. Just prior to collision, another type of magnet is used to "squeeze" the particles closer together to increase the chances of collisions. The particles are so tiny that the task of making them collide is akin to firing two needles 10 kilometers apart with such precision that they meet halfway.

All the controls for the accelerator, its services and technical infrastructure are housed under one roof at the CERN Control Centre. From here, the beams inside the LHC are made to collide at four locations around the accelerator ring, corresponding to the positions of four particle detectors – ATLAS, CMS, ALICE and LHCb. The CMS magnet system contains about 100,000 times stronger magnetic field than that is of Earth and have iron more than in the Eiffel Tower [6].

What is the data flow from the LHC experiments?

The CERN Data Centre stores more than 30 petabytes of data per year from the LHC experiments, enough to fill about 1.2 million Blu-ray discs, i.e. 250 years of HD video. Over 100 petabytes of data are permanently archived, on tape. The data recorded by each of the big experiments at the LHC will be enough to fill around 100000 DVDs every year [6].

What is the LHC power consumption?

The total power consumption of the LHC (and experiments) is equivalent to 600 GWh per year, with a maximum of 650 GWh in 2012 when the LHC was running at 4 TeV. For Run 2 (post-shutdown second run of LHC), the estimated power consumption is 750 GWh per year.

The total CERN energy consumption is 1.3 TWh per year while the total electrical energy production in the world is around 20000 TWh, in the European Union 3400 TWh, in France around 500 TWh, and in Geneva canton 3 TWh.

Upgrades

The LHC is planned to run over the next 20 years, with several stops scheduled for upgrades and maintenance work. The run 2 has just been finished last year and until end of 2020 or early 2021 the long shutdown remains for for further upgrades and preparation for run 3. LHC has plan to continue few more runs until 2035, the plan is as below [16].